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-(* \sqrt 2 is irrationnal, (c) 2006 Pierre Corbineau *)
-
-Set Firstorder Depth 1.
-Require Import ArithRing Wf_nat Peano_dec Div2 Even Lt.
-
-Lemma double_div2: forall n, div2 (double n) = n.
-proof.
- assume n:nat.
- per induction on n.
- suppose it is 0.
- suffices (0=0) to show thesis.
- thus thesis.
- suppose it is (S m) and Hrec:thesis for m.
- have (div2 (double (S m))= div2 (S (S (double m)))).
- ~= (S (div2 (double m))).
- thus ~= (S m) by Hrec.
- end induction.
-end proof.
-Show Script.
-Qed.
-
-Lemma double_inv : forall n m, double n = double m -> n = m .
-proof.
- let n, m be such that H:(double n = double m).
-have (n = div2 (double n)) by double_div2,H.
- ~= (div2 (double m)) by H.
- thus ~= m by double_div2.
-end proof.
-Qed.
-
-Lemma double_mult_l : forall n m, (double (n * m)=n * double m).
-proof.
- assume n:nat and m:nat.
- have (double (n * m) = n*m + n * m).
- ~= (n * (m+m)) by * using ring.
- thus ~= (n * double m).
-end proof.
-Qed.
-
-Lemma double_mult_r : forall n m, (double (n * m)=double n * m).
-proof.
- assume n:nat and m:nat.
- have (double (n * m) = n*m + n * m).
- ~= ((n + n) * m) by * using ring.
- thus ~= (double n * m).
-end proof.
-Qed.
-
-Lemma even_is_even_times_even: forall n, even (n*n) -> even n.
-proof.
- let n be such that H:(even (n*n)).
- per cases of (even n \/ odd n) by even_or_odd.
- suppose (odd n).
- hence thesis by H,even_mult_inv_r.
- end cases.
-end proof.
-Qed.
-
-Lemma main_thm_aux: forall n,even n ->
-double (double (div2 n *div2 n))=n*n.
-proof.
- given n such that H:(even n).
- *** have (double (double (div2 n * div2 n))
- = double (div2 n) * double (div2 n))
- by double_mult_l,double_mult_r.
- thus ~= (n*n) by H,even_double.
-end proof.
-Qed.
-
-Require Import Omega.
-
-Lemma even_double_n: (forall m, even (double m)).
-proof.
- assume m:nat.
- per induction on m.
- suppose it is 0.
- thus thesis.
- suppose it is (S mm) and thesis for mm.
- then H:(even (S (S (mm+mm)))).
- have (S (S (mm + mm)) = S mm + S mm) using omega.
- hence (even (S mm +S mm)) by H.
- end induction.
-end proof.
-Qed.
-
-Theorem main_theorem: forall n p, n*n=double (p*p) -> p=0.
-proof.
- assume n0:nat.
- define P n as (forall p, n*n=double (p*p) -> p=0).
- claim rec_step: (forall n, (forall m,m<n-> P m) -> P n).
- let n be such that H:(forall m : nat, m < n -> P m) and p:nat .
- per cases of ({n=0}+{n<>0}) by eq_nat_dec.
- suppose H1:(n=0).
- per cases on p.
- suppose it is (S p').
- assume (n * n = double (S p' * S p')).
- =~ 0 by H1,mult_n_O.
- ~= (S ( p' + p' * S p' + S p'* S p'))
- by plus_n_Sm.
- hence thesis .
- suppose it is 0.
- thus thesis.
- end cases.
- suppose H1:(n<>0).
- assume H0:(n*n=double (p*p)).
- have (even (double (p*p))) by even_double_n .
- then (even (n*n)) by H0.
- then H2:(even n) by even_is_even_times_even.
- then (double (double (div2 n *div2 n))=n*n)
- by main_thm_aux.
- ~= (double (p*p)) by H0.
- then H':(double (div2 n *div2 n)= p*p) by double_inv.
- have (even (double (div2 n *div2 n))) by even_double_n.
- then (even (p*p)) by even_double_n,H'.
- then H3:(even p) by even_is_even_times_even.
- have (double(double (div2 n * div2 n)) = n*n)
- by H2,main_thm_aux.
- ~= (double (p*p)) by H0.
- ~= (double(double (double (div2 p * div2 p))))
- by H3,main_thm_aux.
- then H'':(div2 n * div2 n = double (div2 p * div2 p))
- by double_inv.
- then (div2 n < n) by lt_div2,neq_O_lt,H1.
- then H4:(div2 p=0) by (H (div2 n)),H''.
- then (double (div2 p) = double 0).
- =~ p by even_double,H3.
- thus ~= 0.
- end cases.
- end claim.
- hence thesis by (lt_wf_ind n0 P).
-end proof.
-Qed.
-
-Require Import Reals Field.
-(*Coercion INR: nat >->R.
-Coercion IZR: Z >->R.*)
-
-Open Scope R_scope.
-
-Lemma square_abs_square:
- forall p,(INR (Z.abs_nat p) * INR (Z.abs_nat p)) = (IZR p * IZR p).
-proof.
- assume p:Z.
- per cases on p.
- suppose it is (0%Z).
- thus thesis.
- suppose it is (Zpos z).
- thus thesis.
- suppose it is (Zneg z).
- have ((INR (Z.abs_nat (Zneg z)) * INR (Z.abs_nat (Zneg z))) =
- (IZR (Zpos z) * IZR (Zpos z))).
- ~= ((- IZR (Zpos z)) * (- IZR (Zpos z))).
- thus ~= (IZR (Zneg z) * IZR (Zneg z)).
- end cases.
-end proof.
-Qed.
-
-Definition irrational (x:R):Prop :=
- forall (p:Z) (q:nat),q<>0%nat -> x<> (IZR p/INR q).
-
-Theorem irrationnal_sqrt_2: irrational (sqrt (INR 2%nat)).
-proof.
- let p:Z,q:nat be such that H:(q<>0%nat)
- and H0:(sqrt (INR 2%nat)=(IZR p/INR q)).
- have H_in_R:(INR q<>0:>R) by H.
- have triv:((IZR p/INR q* INR q) =IZR p :>R) by * using field.
- have sqrt2:((sqrt (INR 2%nat) * sqrt (INR 2%nat))= INR 2%nat:>R) by sqrt_def.
- have (INR (Z.abs_nat p * Z.abs_nat p)
- = (INR (Z.abs_nat p) * INR (Z.abs_nat p)))
- by mult_INR.
- ~= (IZR p* IZR p) by square_abs_square.
- ~= ((IZR p/INR q*INR q)*(IZR p/INR q*INR q)) by triv. (* we have to factor because field is too weak *)
- ~= ((IZR p/INR q)*(IZR p/INR q)*(INR q*INR q)) using ring.
- ~= (sqrt (INR 2%nat) * sqrt (INR 2%nat)*(INR q*INR q)) by H0.
- ~= (INR (2%nat * (q*q))) by sqrt2,mult_INR.
- then (Z.abs_nat p * Z.abs_nat p = 2* (q * q))%nat.
- ~= ((q*q)+(q*q))%nat.
- ~= (Div2.double (q*q)).
- then (q=0%nat) by main_theorem.
- hence thesis by H.
-end proof.
-Qed.